Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=51912

Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=51912
Fix several small issues in Header processing in Nio HTTP connector:
- Split HEADER_VALUE state into HEADER_VALUE_START and HEADER_VALUE.
The first one skips leading whitespace, the second one - does not.
- Trim trailing tabs as well as spaces. RFC-2616 Ch.4.2 says that both are LWS and both can be ignored.
- Always trim whitespace from last line.
- Fix initial value of lastSignificantChar when calling skipLine(). It fixes BZ 51912.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1177160 13f79535-47bb-0310-9956-ffa450edef68