[generator] Fix <remove-attr/> metadata (#999)

Fixes: #976

Given an `api.xml` element like:

	<package name='my.package'>
	  <class name='MyClass' api-since='29' />
	</package>

One would expect to be able to remove the `class/@api-since` attribute
with this metadata:

	<remove-attr path='/api/package[@name='my.package']/class[@name='MyClass']' name='api-since' />

However, even though `generator` reads the `name` attribute, it then
proceeds to ignore it and remove *all* attributes on the matched node,
resulting in:

	<package name='my.package'>
	  <class />
	</package>

This seems unintuitive, and I'm not sure how this could ever be
successfully used in a binding project, since leaving an empty
`<class/>`/etc. node causes `generator` to crash.

Update `<remove-attr/>` to work as expected, removing the named
attribute.

src/Java.Interop.Tools.Generator/Metadata/FixupXmlDocument.cs

@@ -153,7 +153,7 @@ public void Apply (ApiXmlDocument apiDocument, string apiLevelString, int produc
 						var matched = false;
 
 						foreach (var node in nodes) {
-							node.RemoveAttributes ();
+							node.Attributes (name).Remove ();
 							matched = true;
 						}
 

tests/generator-Tests/Unit-Tests/FixupXmlDocumentTests.cs

@@ -82,7 +82,19 @@ public void RemoveAttribute ()
 
 			api.ApplyFixupFile (fixup);
 
-			Assert.AreEqual ("<api><package /><package name='java' jni-name='java' /></api>", api.ApiDocument.ToString (SaveOptions.DisableFormatting).Replace ('\"', '\''));
+			Assert.AreEqual ("<api><package jni-name='android' /><package name='java' jni-name='java' /></api>", api.ApiDocument.ToString (SaveOptions.DisableFormatting).Replace ('\"', '\''));
+		}
+
+		[Test]
+		public void RemoveNotFoundAttribute ()
+		{
+			// Attribute 'foo' doesn't exist on node
+			var api = GetXmlApiDocument ();
+			var fixup = GetFixupXmlDocument ("<remove-attr path=\"/api/package[@name='android']\" name='foo' />");
+
+			api.ApplyFixupFile (fixup);
+
+			Assert.AreEqual ("<api><package name='android' jni-name='android' /><package name='java' jni-name='java' /></api>", api.ApiDocument.ToString (SaveOptions.DisableFormatting).Replace ('\"', '\''));
 		}
 
 		[Test]