Add workspace/configuration handler that recognizes insertSpaces/tabSize

If using a eclipse.jdt.ls version that includes
eclipse-jdtls/eclipse.jdt.ls#1657 this will finally
make code-actions respect the clients shiftwidth/expandtab options and
no longer include tabs when the user uses spaces.

The alternative to overriding the handler would have been to document
that users should include these options in their `config.settings`, but
given that there is no good reason that I can think of where you don't
want to respond with your local editor settings it seems justified to
include this out of the box instead of requiring every user to add this.

lua/jdtls/setup.lua

@@ -53,6 +53,34 @@ local extendedClientCapabilities = {
 };
 
 
+local function tabsize(bufnr)
+  local get_option = api.nvim_buf_get_option
+  local sts = get_option(bufnr, 'softtabstop')
+  return (
+    (sts > 0 and sts)
+    or (sts < 0 and get_option(bufnr, 'shiftwidth'))
+    or get_option(bufnr, 'tabstop')
+  )
+end
+
+
+local function configuration_handler(err, method, params, client_id, bufnr, config)
+  local client = vim.lsp.get_client_by_id(client_id)
+  -- This isn't done in start_or_attach because a user could use a plugin like editorconfig to configue tabsize/spaces
+  -- That plugin may run after `start_or_attach` which is why we defer the setting lookup.
+  -- This ensures the language-server will use the latest version of the options
+  client.config.settings = vim.tbl_deep_extend('keep', client.config.settings or {}, {
+    java = {
+      format = {
+        insertSpaces = api.nvim_buf_get_option(bufnr, 'expandtab'),
+        tabSize = tabsize(bufnr),
+      }
+    }
+  })
+  return vim.lsp.handlers['workspace/configuration'](err, method, params, client_id, bufnr, config)
+end
+
+
 local function start_or_attach(config)
   assert(config, 'config is required')
   assert(
@@ -82,6 +110,7 @@ local function start_or_attach(config)
   )
   config.handlers = config.handlers or {}
   config.handlers['language/status'] = config.handlers['language/status'] or status_callback
+  config.handlers['workspace/configuration'] = config.handlers['workspace/configuration'] or configuration_handler
   config.capabilities = config.capabilities or lsp.protocol.make_client_capabilities()
   config.capabilities.textDocument.codeAction = {
       dynamicRegistration = false;