fix: increase default receive buffer to 16kb (#1814)

Since TLS packets have a maximum size of 16kb it makes
sense to fully accommodate them on reads.

Co-authored-by: Rusty Conover <rusty@Juno.local>

httplib.h

@@ -91,7 +91,7 @@
 #endif
 
 #ifndef CPPHTTPLIB_RECV_BUFSIZ
-#define CPPHTTPLIB_RECV_BUFSIZ size_t(4096u)
+#define CPPHTTPLIB_RECV_BUFSIZ size_t(16384u)
 #endif
 
 #ifndef CPPHTTPLIB_COMPRESSION_BUFSIZ