feat: filter false props before generating styles

Prevents generating `undefined` style properties e.g

`{ fontSize: '16px', fontFamily: undefined }`

The above props would generate the following styles:

```
font-size: 16px;
font-family: undefined;
```

The new feature is filtering false props before generating and injecting the styles. The new styles now look something like this:

```
font-size: 16px;
```

More details: styled-components#102

Author: andreruffert

src/models/StyledComponent.js

@@ -1,5 +1,6 @@
 import css from '../constructors/css'
 import normalizeProps from '../utils/normalizeProps'
+import removeFalseProps from '../utils/removeFalseProps'
 import isVueComponent from '../utils/isVueComponent'
 
 export default (ComponentStyle) => {
@@ -68,7 +69,10 @@ export default (ComponentStyle) => {
       },
       computed: {
         generatedClassName () {
-          const componentProps = { theme: this.theme, ...this.$props }
+          const componentProps = {
+            theme: this.theme,
+            ...removeFalseProps(this.$props)
+          }
           return this.generateAndInjectStyles(componentProps)
         },
         theme () {