enhance debugging output in class-IXR.php

[wnorris]

this patch will add the classes 'ixr_request' and 'ixr_response' to the <pre> elements printed by IXR_Client when debug mode is enabled. It also retains the response headers and whitespace for the debug response content that is printed to screen.

[westi]

(In [10423]) Make the IXR debug classy. See #8916 props wnorris

[wnorris]

oops, looks like I forgot to initialize $debug_contents, causing a PHP notice. new patch attached.

[westi]

(In [10436]) Setup an initial value to avoid a Notice. See #8916 props wnorris.