Override indent-tabs-mode for code sent to Python.

It was possible that a globally configured indent-tabs-mode would
cause tabs to be inserted into code before it was sent to a Python
shell, confusing the interpreter. We now override the variable with
a correct local default.

Tabs are evil.

Fixes #499

elpy.el

@@ -1494,7 +1494,8 @@ code is executed."
   (if (= beg end)
       ""
     (let ((region (buffer-substring beg end))
-          (indent-level nil))
+          (indent-level nil)
+          (indent-tabs-mode nil))
       (with-temp-buffer
         (insert region)
         (goto-char (point-min))

test/elpy-shell--region-without-indentation-test.el

@@ -47,4 +47,13 @@
               "\n"
               "meh\n")
       (should-error (elpy-shell--region-without-indentation (point-min)
-                                                            (point-max))))))
+                                                            (point-max))))
+
+    (with-temp-buffer
+      (insert "if True:\n"
+              "    if True:\n"
+              "        pass\n")
+      (should (equal (let ((indent-tabs-mode t))
+                       (elpy-shell--region-without-indentation (point-min)
+                                                               (point-max)))
+                     (buffer-string))))))