Recognize Content-Type as special header in MHSRB

When adding headers generically, MockHttpServletRequestBuilder now
recognizes Content-Type and updates the contentType field accordingly.

Issue: SPR-11308
Stop using deprecated junit.framework.Assert class
Trim tokenized strings in websocket namespace

Issue: SPR-11307
Fixed a typo in RequestMappingHandlerMapping.java
Add explanation for the special 'default' profile

Better integrated explanation of the 'default' profile with both XML
and Java config examples showing the same thing.

Issue: SPR-11256
Fix typo in documentation

Issue: SPR-11301
Polish explanation of the 'default' profile

Issue: SPR-11256
Add UpgradeRequestStrategy for WildFly/Undertow

Issue: SPR-11237