Bugfix: Change "enabled" to "enable" to conform to Sendgrid API. (#624)

# Problem

[A recent PR](#618) added click tracking support for the SendGrid adapter. Unfortunately, we didn't catch a typo that sends "enabled" as the param instead of "enable" as is recognized by the API. This results in the following error:

```
"{\"errors\":[{\"message\":\"The click_tracking enable parameter is required.\",\"field\":\"tracking_settings.click_tracking.enable\",\"help\":\"http://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/errors.html#message.tracking_settings.click_tracking.enable\"}]}"
``` 

with these params:

```
"tracking_settings" => %{"click_tracking" => %{"enable_text" => false, "enabled" => false}
```

Where it says `"enabled" => false` in the params, it should say `"enable" => false`

# Solution

Fix the typo and update the tests.

lib/bamboo/adapters/send_grid_adapter.ex

@@ -360,7 +360,7 @@ defmodule Bamboo.SendGridAdapter do
     tracking_settings =
       body
       |> Map.get(:tracking_settings, %{})
-      |> Map.put(:click_tracking, %{enabled: enabled, enable_text: enabled})
+      |> Map.put(:click_tracking, %{enable: enabled, enable_text: enabled})
 
     body
     |> Map.put(:tracking_settings, tracking_settings)

test/lib/bamboo/adapters/send_grid_adapter_test.exs

@@ -347,7 +347,7 @@ defmodule Bamboo.SendGridAdapterTest do
     |> SendGridAdapter.deliver(@config)
 
     assert_receive {:fake_sendgrid, %{params: params}}
-    assert params["tracking_settings"]["click_tracking"]["enabled"] == true
+    assert params["tracking_settings"]["click_tracking"]["enable"] == true
     assert params["tracking_settings"]["click_tracking"]["enable_text"] == true
   end
 
@@ -363,7 +363,7 @@ defmodule Bamboo.SendGridAdapterTest do
     |> SendGridAdapter.deliver(@config)
 
     assert_receive {:fake_sendgrid, %{params: params}}
-    assert params["tracking_settings"]["click_tracking"]["enabled"] == false
+    assert params["tracking_settings"]["click_tracking"]["enable"] == false
     assert params["tracking_settings"]["click_tracking"]["enable_text"] == false
   end