Merge capabilities correctly.

* Fixes some completion snippets not working properly, especially
  functions w/o arguments would not complete the empty parenthesis.
* Thanks @wookayin via hrsh7th/cmp-nvim-lsp#38 (comment)

lua/sd/lsp/init.lua

@@ -11,9 +11,11 @@ util.on_setup = util.add_hook_after(util.on_setup, function(config)
     else
         config.on_attach = on_attach
     end
-    local cnl = require 'cmp_nvim_lsp'
-    config.capabilities = vim.lsp.protocol.make_client_capabilities()
-    config.capabilities.completion = cnl.default_capabilities(config.capabilities).completion
+    config.capabilities = vim.tbl_deep_extend(
+        'force',
+        vim.lsp.protocol.make_client_capabilities(),
+        require('cmp_nvim_lsp').default_capabilities()
+    )
 end)
 
 require('mason-lspconfig').setup_handlers {