Set the host header if not set in CRT HTTP client.

Author: alextwoods

packages/smithy-http/src/smithy_http/aio/crt.py

@@ -293,7 +293,9 @@ async def _marshal_request(
         :py:class:`smithy_http.aio.HTTPRequest`"""
         headers_list = []
         if "Host" not in request.fields:
-            request.fields.set_field(Field(name="Host", values=[request.destination.host]))
+            request.fields.set_field(
+                Field(name="Host", values=[request.destination.host])
+            )
 
         for fld in request.fields.entries.values():
             # TODO: Use literal values for "header"/"trailer".

packages/smithy-http/src/smithy_http/interfaces/__init__.py

@@ -98,6 +98,10 @@ def __len__(self) -> int:
         """Get total number of Field entries."""
         ...
 
+    def __contains__(self, key: str) -> bool:
+        """Allows in/not in checks on Field entries."""
+        ...
+
     def get_by_type(self, kind: FieldPosition) -> list[Field]:
         """Helper function for retrieving specific types of fields.