Fix -wConversion warnings #748 (#754)

* Fix -wConversion warning in async.h #748

Fix a warning about problematic conversion from 'long int' to 'int'
appearing when compiling http.cc with -wConversion flag

* Suppress -wConversion warnings in date.h #748

Suppress warnings originating in date.h. The similar suppression appears
in the newer version of date.h appearing in its original repository
https://github.com/HowardHinnant/dateCo-authored-by: Michael Tseitlin <michael.tseitlin@concertio.com>

src/common/http.cc

@@ -794,7 +794,7 @@ Async::Promise<ssize_t> ResponseWriter::putOnWire(const char *data,
     auto fd = peer()->fd();
 
     return transport_->asyncWrite(fd, buffer)
-        .then<std::function<Async::Promise<ssize_t>(int)>,
+        .then<std::function<Async::Promise<ssize_t>(ssize_t)>,
               std::function<void(std::exception_ptr &)>>(
             [=](int /*l*/) {
               return Async::Promise<ssize_t>([=](

src/common/http_defs.cc

@@ -8,7 +8,14 @@
 #include <iostream>
 
 #include <pistache/common.h>
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wconversion"
+#endif
 #include <pistache/date.h>
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
 #include <pistache/http_defs.h>
 
 namespace Pistache {