fix undefined mixin value cases

Author: raveclassic

src/components/themr.js

@@ -207,6 +207,7 @@ function merge(original = {}, mixin = {}) {
         break
       }
 
+      case 'undefined': //fallthrough - handles accidentally unset values which may come from props
       case 'function': {
         //this handles issue when isomorphic-style-loader addes helper functions to css-module
         break //just skip

test/components/themr.spec.js

@@ -607,4 +607,16 @@ describe('themeable function', () => {
     const result = themeable(a, b, c)
     expect(result).toEqual(expected)
   })
+
+  it('should skip undefined mixin values', () => {
+    const a = {
+      test: 'a'
+    }
+    const b = {
+      test: undefined
+    }
+    const expected = a
+    const result = themeable(a, b)
+    expect(result).toEqual(expected)
+  })
 })