Add deprecated put(Object key, Object value) to RequestContext to provide binary compatibility with 2.7

The solution for https://issues.sonatype.org/browse/NEXUS-6091 implemented in https://github.com/sonatype/nexus-oss/commit/ae197f5b97b97d6e289a2b24c3feda2467cd0c3d accidentally introduced a binary incompatibility for clients using RequestContext.put

This can be fixed by adding a deprecated `put(Object key, Object value)` method for old clients.

http://bamboo.s/browse/NX-OSSF2-1 :white_check_mark: