Consistent casing for default headers Accept and Content-Type

Author: glinf

src/mcp/client/streamable_http.py

@@ -42,7 +42,7 @@
 MCP_SESSION_ID = "mcp-session-id"
 MCP_PROTOCOL_VERSION = "mcp-protocol-version"
 LAST_EVENT_ID = "last-event-id"
-CONTENT_TYPE = "content-type"
+CONTENT_TYPE = "Content-Type"
 ACCEPT = "Accept"