The regex that parses header lines potentially causes an unlimited
amount of backtracking, which can cause an exception in the libc++ regex
engine.
The exception that occurs looks like this and is identical to the
message of the exception fixed in
https://github.com/yhirose/cpp-httplib/pull/280:
libc++abi.dylib: terminating with uncaught exception of type
std::__1::regex_error: The complexity of an attempted match
against a regular expression exceeded a pre-set level.
This commit eliminates the problematic backtracking.
bf7700d192
