Include ALL source artifacts in published sources jars

This commit updates the Gradle build to ensure that *all* source
artifacts are including in published 'sources' jars.

To achieve this, we are now assembling 'sources' jars from
'sourceSets.main.allSource' and not explicitly including or excluding
anything by default.

Issue: SPR-12085
Verify custom MIME type support in MockServletContext
How to register MIME types in MockServletContext

Prior to this commit, it was unclear that it was possible to register
custom MIME types when using MockServletContext.

This commit updates the Javadoc for MockServletContext.getMimeType()
with an example of how to achieve this using the MimetypesFileTypeMap
from the Java Activation Framework.

Issue: SPR-12126