I'm using the HTTP post version of the Stratum protocol and content type "application/stratum".
The HTTP version of stratum isn't supported in the later server versions, it's unlikely you'll be able to use them in any sort of capacity in the future. When connecting to the TCP+SSL version of electrum-server your command works perfectly, though the block height field in your example should be an integer and not a string.
telnet electrum.be 50001
Trying 88.198.241.196...
Connected to electrum.be.
Escape character is '^]'.
{ "id": 1, "method": "blockchain.transaction.get_merkle", "params": [ "967d009ee103340d6762819ebf452107561423fe97b04fbf501594f231e212c4", 369863 ] }
{"id": 1, "result": {"merkle": ["d844420b0f01398953b809b844bc9a5987f41d1373dab3180b6b4fe4de8633c4", "3c1f63dae13e84aaba94d6ee12c7b48fa7b470eacfbe6e183ba008b7cbc3725c", "0c5141f62f1ad58a6ebcee98868451ff392bc49bfb6daa5e4f492b6d175812cf", "e414fd7ee39a83b7bd524f98ed09efa279a97fecbeeca68087de384eda71fc31", "47fbf181989f2549d0d7a9fee2337ea60295059df0c0267cca173fd43b8e8596", "bc441c955a6d9bc629e3a8e428aa7670d2bf631a9d11887bb4ad6b534fcf4817", "1985800e12a150e6196ac07119278ea84ad296f1d8b2c88b2c85f61035f6a7cd", "fc02dfab3083ca9185bce4e8b9357b5aacacce2172c969ac543da04eaa1c0c1d", "6a36f10f34da6d66fdf75d4d97e0796106e7594409d354b8419a248231ab6935", "cbc02af67b294fe7bad08a69f9435b520f8f89fa221583e0a84c8d9c9dc469a0", "1043c7a6b9b4c42243e84a49a1f40476692b5d36d2f64dfb14409490ca3dfad2", "fbbaec1ca9d29ba9ee6a89214428556827b4e5a254de59de5adabbabb2621f54"], "pos": 2917, "block_height": 369863}}