Be more generous with our sourcemap discovery

The sourcemap indicator line doesn't have to be the last line in the
document, so it's not too safe to assume so.

jQuery, for example, puts it at the top.
The proposed header is actually just SourceMap, not X-SourceMap

See:
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6f
AH0KY0k/edit#
Sourcemaps allow a prefix to thwart XSSI attacks

See:
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6f
AH0KY0k/edit#heading=h.h7yy76c5il9v
Merge pull request #760 from mattrobenolt/discover-sourcemap

Be more generous with our sourcemap discovery