Removing all keys from ThreadContextMap does not clear the ThreadLocal

[ppkarwasz]

The implementations of ThreadContextMap#remove and CleanableThreadContextMap#removeAll in the DefaultThreadContextMap and CopyOnWriteSortedArrayThreadContextMap implementations do not clear the underlying ThreadLocal, after all keys have been removed from the context map.

This issue prevents CopyOnWriteSortedArrayThreadContextMap to be safely used in a web application, if code like this is used:

ThreadContext.put("key", "value");
try {
    ...
} finally {
    ThreadContext.remove("key");
}

Remark: this issue does not affect the GarbageFreeSortedArrayThreadContextMap implementation, since leaving a non-null value of ThreadContext is part of its garbage-free design.