Backport 4.1 to 4.0 on HttpPostRequestDecoder

Motivation
4.0 was not modified in the same time than 4.1 while the difference was
limited.
Include the fix on "=" character in Boundary.

Issue #3004 shows that "=" character was not supported as it should in
the HttpPostRequestDecoder in form-data boundary.

Modifications:
Backport from 4.1 to 4.0 while respecting interfaces.

Add 2 methods in StringUtil
- split with maxParm argument: String split with max parts only (to prevent multiple '='
to be source of extra split while not needed)
- substringAfter: String part after delimiter (since first part is not
needed)
Use those methods in HttpPostRequestDecoder.
Change and the HttpPostRequestDecoderTest to check using a boundary
beginning with "=".

Results:
Backport done (Issue #2886 fix)
Issue #3004 fix too
The fix implies more stability and fix the relative issues.