This commit moves the features of MockHttpServletRequestBuilder in
an abstract class so that another class can offer the same feature
whilst providing AssertJ support. This wasn't possible previously as
the builder's return type would lose the concrete builder types.
This change benefits MockMultipartHttpServletRequestBuilder that can
use the same mechanism to offer additional settings.
This change also makes it so that a builder instance can be created
using only the HttpMethod. Previously, the URI had to be provided as
well and that makes it impossible to specify it using the builder.
See gh-32913
a3e7fd47c6