V3.9 - Fix "=" character in HttpPostRequestDecoder

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

Modifications:
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:
The fix implies more stability and fix the issue.