Avoid a copy in RPC output

Split up HTTPReply into HTTPReply and HTTPReplyHeader, so that
the message data can be streamed directly.

Also removes a c_str(), which would have prevented binary
output with NUL characters in it.

src/rpcprotocol.cpp

@@ -93,8 +93,7 @@ string HTTPError(int nStatus, bool keepalive, bool headersOnly)
                      headersOnly, "text/plain");
 }
 
-string HTTPReply(int nStatus, const string& strMsg, bool keepalive,
-                 bool headersOnly, const char *contentType)
+string HTTPReplyHeader(int nStatus, bool keepalive, size_t contentLength, const char *contentType)
 {
     return strprintf(
             "HTTP/1.1 %d %s\r\n"
@@ -103,17 +102,25 @@ string HTTPReply(int nStatus, const string& strMsg, bool keepalive,
             "Content-Length: %u\r\n"
             "Content-Type: %s\r\n"
             "Server: bitcoin-json-rpc/%s\r\n"
-            "\r\n"
-            "%s",
+            "\r\n",
         nStatus,
         httpStatusDescription(nStatus),
         rfc1123Time(),
         keepalive ? "keep-alive" : "close",
-        (headersOnly ? 0 : strMsg.size()),
+        contentLength,
         contentType,
-        FormatFullVersion(),
-        (headersOnly ? "" : strMsg.c_str())
-        );
+        FormatFullVersion());
+}
+
+string HTTPReply(int nStatus, const string& strMsg, bool keepalive,
+                 bool headersOnly, const char *contentType)
+{
+    if (headersOnly)
+    {
+        return HTTPReplyHeader(nStatus, keepalive, 0, contentType);
+    } else {
+        return HTTPReplyHeader(nStatus, keepalive, strMsg.size(), contentType) + strMsg;
+    }
 }
 
 bool ReadHTTPRequestLine(std::basic_istream<char>& stream, int &proto,

src/rpcprotocol.h

@@ -143,6 +143,8 @@ private:
 std::string HTTPPost(const std::string& strMsg, const std::map<std::string,std::string>& mapRequestHeaders);
 std::string HTTPError(int nStatus, bool keepalive,
                       bool headerOnly = false);
+std::string HTTPReplyHeader(int nStatus, bool keepalive, size_t contentLength,
+                      const char *contentType = "application/json");
 std::string HTTPReply(int nStatus, const std::string& strMsg, bool keepalive,
                       bool headerOnly = false,
                       const char *contentType = "application/json");

src/rpcserver.cpp

@@ -862,7 +862,7 @@ static bool HTTPReq_JSONRPC(AcceptedConnection *conn,
         else
             throw JSONRPCError(RPC_PARSE_ERROR, "Top-level object parse error");
 
-        conn->stream() << HTTPReply(HTTP_OK, strReply, fRun) << std::flush;
+        conn->stream() << HTTPReplyHeader(HTTP_OK, fRun, strReply.size()) << strReply << std::flush;
     }
     catch (Object& objError)
     {