Add header name to ex. msg in HeaderResultMatchers

Prior to this commit, the longValue() in HeaderResultMatchers added the
expected response header name to generated assertion failure messages;
however, both string() methods did not. This made it more difficult to
analyze the cause of failed tests.

Furthermore, while investigating a solution for this issue it became
apparent that longValue() throws a NullPointerException if the response
does not contain the specified header.

This commit addresses these issues as follows:

 - All methods in HeaderResultMatchers now include the response header
   name in generated assertion failure messages.

 - HeaderResultMatchers.longValue() now avoids NullPointerExceptions by
   explicitly asserting that the response contains the specified header.

 - The unit tests in HeaderAssertionTests have been expanded to test
   most foreseeable use cases.

Issue: SPR-10659