ext/thin_parser: set LC_CTYPE to C

thin fails in unexpected ways when Accept-Language is equal to 'tr' due
to the fact that (e.g.) Turkish locale affects functions like
toupper()/tolower().

Problem lies in the fact that Turkish has different case mappings for
letters "I" and "i". A detailed explanation of the "problem" can be
found at: http://www.i18nguy.com/unicode/turkish-i18n.html

This commit sets LC_CTYPE locale to "C" which is used by functions like
toupper() and tolower().