spec/: use 2048 bit RSA keys for tests

Some OpenSSL libraries have smaller key sizes disabled.
Failure looks like:
```
ssl.context:setCertificate: ssl_rsa.c:310:error:140AB18F:SSL routines:SSL_CTX_use_certificate:ee key too small
```

spec/client_spec.lua

@@ -52,7 +52,7 @@ describe("http.client module", function()
 		s:close()
 	end
 	local function new_server_ctx()
-		local key = openssl_pkey.new()
+		local key = openssl_pkey.new({type="RSA", bits=2048})
 		local crt = openssl_x509.new()
 		crt:setPublicKey(key)
 		crt:sign(key)

spec/tls_spec.lua

@@ -28,7 +28,7 @@ describe("http.tls module", function()
 			ctx:setCipherList(banned_cipher_list)
 			ctx:setEphemeralKey(openssl_pkey.new{ type = "EC", curve = "prime256v1" })
 			local crt = openssl_x509.new()
-			local key = openssl_pkey.new()
+			local key = openssl_pkey.new({type="RSA", bits=2048})
 			crt:setPublicKey(key)
 			crt:sign(key)
 			assert(ctx:setPrivateKey(key))