Support implicit ports in MHSR.getRequestURL()

Prior to this commit, MockHttpServletRequest.getRequestURL() always
included the server port number in the reconstructed request URL, even
for implicit ports (i.e., 80 and 443) and negative ports.

MockHttpServletRequest.getRequestURL() now omits the port number when
reconstructing a URL that has an implicit or negative port.

Issue: SPR-9726