Consistent casing for default headers Accept and Content-Type (#1263)

Author: glinf

src/mcp/client/streamable_http.py

@@ -43,7 +43,7 @@
 MCP_PROTOCOL_VERSION = "mcp-protocol-version"
 LAST_EVENT_ID = "last-event-id"
 CONTENT_TYPE = "content-type"
-ACCEPT = "Accept"
+ACCEPT = "accept"
 
 
 JSON = "application/json"