Script bin/generate_test_tt2.pl is not able to generate proper regexp
from a mixed Accept header field like for example:
Accept: application/pidf+xml

In this case there is no substitution and the final .tt2 file
does not escape the `+' sign causing the following test to fail
even if user agent are sendind the correct header.
Change-Id: I2a1d66d3482a8c87d9bf09bb2665951aa42c4722