Don't split too aggressively with named assigns

If a function call that has a named assign is part of an argument list,
don't cause the other arguments to split just because it has a named
assign. I.e., 'a' won't be forced to split here:

  func(a, b, c, d(x, y, z=42))
diff --git a/CHANGELOG b/CHANGELOG
index 47f9f03..1b7e55e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -2,6 +2,14 @@
 # All notable changes to this project will be documented in this file.
 # This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [0.15.2] UNRELEASED
+### Fixed
+- Don't perform a global split when a named assign is part of a function call
+  which itself is an argument to a function call. I.e., don't cause 'a' to
+  split here:
+
+      func(a, b, c, d(x, y, z=42))
+
 ## [0.15.1] 2017-01-21
 ### Fixed
 - Don't insert a space between a type hint and the '=' sign.