Remove duplicate variables in assignments

Fixes the compile errors:

    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c: In function 'ngx_http_clojure_merge_loc_conf':
    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c:693:33: error: operation on 'mcf->enable_header_filter' may be undefined [-Werror=sequence-point]
         = mcf->enable_header_filter = mcf->enable_header_filter
                                     ^
    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c:696:34: error: operation on 'conf->enable_header_filter' may be undefined [-Werror=sequence-point]
         = conf->enable_header_filter = conf->enable_header_filter
                                      ^
Fix string comparison

Fixes the compilation errors like:

    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c: In function 'ngx_http_clojure_merge_loc_conf':
    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c:662:78: error: comparison with string literal results in unspecified behavior [-Werror=address]
       if (!conf->handler ##  _type.len && conf->enable_ ## handler &&  # handler != "content_handler") {  \
                                                                                  ^
    ../nginx-clojure-0.3.0/src/c/ngx_http_clojure_module.c:704:2: note: in expansion of macro 'ngx_http_clojure_merge_handler'
      ngx_http_clojure_merge_handler(conf, prev, content_handler);
      ^
Merge branch 'tmatilai-fix-compiler-errors'
strcmp is OK, but constant pointer cmp is enough