Fixed error: ‘ULONG_MAX’ was not declared in this scope on line 1921 (#445)

* Fixed error:
ULONG_MAX is defined in the limits.h header file. Putting #include <climits>
```
httplib.h: In function ‘bool httplib::detail::read_content_chunked(httplib::Stream&, httplib::ContentReceiver)’:
httplib.h:1921:22: error: ‘ULONG_MAX’ was not declared in this scope
     if (chunk_len == ULONG_MAX) { return false; }
                      ^~~~~~~~~
httplib.h:1921:22: note: suggested alternative: ‘_SC_ULONG_MAX’
     if (chunk_len == ULONG_MAX) { return false; }
                      ^~~~~~~~~
                      _SC_ULONG_MAX
```

* Move #include <climits> to after #include <cassert>

httplib.h

@@ -134,6 +134,7 @@ using socket_t = int;
 #include <array>
 #include <atomic>
 #include <cassert>
+#include <climits>
 #include <condition_variable>
 #include <errno.h>
 #include <fcntl.h>
@@ -174,7 +175,6 @@ inline const unsigned char *ASN1_STRING_get0_data(const ASN1_STRING *asn1) {
 #ifdef CPPHTTPLIB_ZLIB_SUPPORT
 #include <zlib.h>
 #endif
-
 /*
  * Declaration
  */