Allow ApplicationFacade set_config with non-string values

Fixes #388

juju/application.py

@@ -517,7 +517,12 @@ async def set_config(self, config):
         log.debug(
             'Setting config for %s: %s', self.name, config)
 
-        return await app_facade.Set(application=self.name, options=config)
+        str_config = {}
+        for k, v in config.items():
+            if v.get('value') is not None:
+                str_config[k] = str(v.get('value'))
+
+        await app_facade.Set(application=self.name, options=str_config)
 
     async def reset_config(self, to_default):
         """

tests/integration/test_application.py

@@ -46,6 +46,31 @@ async def test_action(event_loop):
         assert 'backup' in actions.keys()
 
 
+@base.bootstrapped
+@pytest.mark.asyncio
+async def test_get_set_config(event_loop):
+    async with base.CleanModel() as model:
+        ubuntu_app = await model.deploy(
+            'percona-cluster',
+            application_name='mysql',
+            series='xenial',
+            channel='stable',
+            config={
+                'tuning-level': 'safest',
+            },
+            constraints={
+                'arch': 'amd64',
+                'mem': 256 * MB,
+            },
+        )
+
+        config = await ubuntu_app.get_config()
+        await ubuntu_app.set_config(config)
+
+        config2 = await ubuntu_app.get_config()
+        assert config == config2
+
+
 @base.bootstrapped
 @pytest.mark.asyncio
 async def test_status_is_not_unset(event_loop):