fix: replacing HeaderMap::new with HeaderMap::with_capacity

[shashitnak]

Summary:
Following improvements were observed when running the nginx benchmark locally

Before the change:

After the change:

Issue Reference(s):
Fixes #... (Replace "..." with the issue number)

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.73%. Comparing base (2b98fde) to head (a93cd03).
Report is 26 commits behind head on perf/base.

❗ Current head a93cd03 differs from pull request most recent head e92a03f

Please upload reports for the commit e92a03f to get more accurate results.

Additional details and impacted files

@@              Coverage Diff              @@
##           perf/base    #1923      +/-   ##
=============================================
- Coverage      89.73%   89.73%   -0.01%     
=============================================
  Files            155      155              
  Lines          15837    15832       -5     
=============================================
- Hits           14212    14207       -5     
  Misses          1625     1625              

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[github-actions]

Bencher

Report	Mon, May 13, 2024 at 14:55:21 UTC
Project	tailcall
Branch	1923/merge
Testbed	benchmarking-runner

🚨 1 ALERT: Threshold Boundary Limit exceeded!

Benchmark	Measure (units)	View	Value	Lower Boundary	Upper Boundary
input/vars.missing	Latency (nanoseconds (ns))	🚨 (view plot | view alert)	11.23 (+37.60%)		9.34 (120.21%)

Click to view all benchmark results

Benchmark	Latency	Latency Results nanoseconds (ns) | (Δ%)	Latency Upper Boundary nanoseconds (ns) | (%)
group_by	✅ (view plot)	544.96 (-10.11%)	636.62 (85.60%)
input/args.missing	✅ (view plot)	22.77 (-8.93%)	27.15 (83.88%)
input/args.nested.existing	✅ (view plot)	40.40 (-24.50%)	65.67 (61.52%)
input/args.nested.missing	✅ (view plot)	35.94 (-7.00%)	40.76 (88.18%)
input/args.root	✅ (view plot)	35.75 (-28.69%)	61.92 (57.74%)
input/headers.existing	✅ (view plot)	32.68 (+2.77%)	33.34 (98.03%)
input/headers.missing	✅ (view plot)	31.57 (+2.76%)	32.83 (96.16%)
input/value.missing	✅ (view plot)	23.08 (-2.06%)	25.28 (91.28%)
input/value.nested.existing	✅ (view plot)	41.13 (-0.64%)	43.55 (94.45%)
input/value.nested.missing	✅ (view plot)	37.63 (+3.64%)	38.24 (98.42%)
input/value.root	✅ (view plot)	37.29 (-3.51%)	40.03 (93.14%)
input/vars.existing	✅ (view plot)	7.14 (-13.51%)	9.02 (79.15%)
input/vars.missing	🚨 (view plot | view alert)	11.23 (+37.60%)	9.34 (120.21%)
test_batched_body	✅ (view plot)	2,589.80 (-99.74%)	2,762,825.90 (0.09%)
test_batched_body #2	✅ (view plot)	1,776,700.00 (+0.75%)	1,891,747.93 (93.92%)
test_data_loader	✅ (view plot)	463,790.00 (-2.04%)	490,463.34 (94.56%)
test_handle_request	✅ (view plot)	172,750.00 (-0.07%)	182,486.35 (94.66%)
with_mustache_expressions	✅ (view plot)	1,155.10 (-1.64%)	1,237.75 (93.32%)
with_mustache_literal	✅ (view plot)	704.86 (-2.56%)	765.85 (92.04%)

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