Relax Content-Type parameter parsing

Unfortunately Opera 12 (this could possibly be found in files generated by
other products as well, or by hand) and below does not write out its
Content-Type parameters according to the standard (start parameter contains
tspecials without being an unquoted-string) in some MHTML files. This patch
relaxes the parsing of Content-Type parameters to allow this.

R=tony@chromium.org
BUG=274407

Review URL: https://chromiumcodereview.appspot.com/23043002

git-svn-id: svn://svn.chromium.org/blink/trunk@158314 bbb929c8-8fbe-4397-9dbb-9b2b20218538