Do not call blocking content property and lazily load response on first access

This reverts commit 7ce89a0 and improves it

Author: neiser

Diff for: locust/contrib/fasthttp.py

@@ -578,7 +578,8 @@ class ResponseContextManager(FastResponse):
     def __init__(self, response, environment, request_meta):
         # copy data from response to this object
         self.__dict__ = response.__dict__
-        self._cached_content = response.content
+        if response._cached_content:
+            self._cached_content = response._cached_content
         # store reference to locust Environment
         self._environment = environment
         self.request_meta = request_meta