Repository: styled-system/styled-system (js)

Issue:
Breakpoints bug when using numbers of different length
Hello hello,
I've encountered a weird behavior when dealing with breakpoints as objects. Regarding your documentation I came up with the following breakpoints within a breakpoints.js file:

```
const breakpoints = [
  '21em', // 336px
  '35em', // 560px
  '45em', // 720px
  '64em', // 1024px
  '75em', // 1200px
  '90em', // 1400px
  '100em', // 1600px
];

breakpoints.phone_portrait = breakpoints[0];
breakpoints.phone_landscape = breakpoints[1];
breakpoints.tablet_portrait = breakpoints[2];
breakpoints.tablet_landscape = breakpoints[3];
breakpoints.desktop_s = breakpoints[4];
breakpoints.desktop_m = breakpoints[5];
breakpoints.desktop_l = breakpoints[6];

export default breakpoints;
```

Afterwards I'm pulling them into the theme.js file. So far so good but once I try to reference them as follows: 


```
H1.defaultProps = {
  fontFamily: 'teko_bold',
  fontWeight: 'normal',
  fontSize: {
    _: 1,
    phone_portrait: 2,
    phone_landscape: 3,
    tablet_portrait: 4,
    tablet_landscape: 5,
    desktop_s: 6,
    desktop_m: 7,
    desktop_l: 9,
  },
};
```

**things get stuck at `90em` - never reaching `100em`**. (The numbers for the fontsizes are just referencing the position of the actual value within the theme.js array for fontSizes)

However, if I prefix all of the values within breakpoints.js so the number will have the same length:


```
const breakpoints = [
  '021em', // 336px
  '035em', // 560px
  '045em', // 720px
  '064em', // 1024px
  '075em', // 1200px
  '090em', // 1400px
  '100em', // 1600px
];
```

**Everything works as expected and even the `100em` breakpoint gets picked up correctly**. But it's a weird thing to do...



Pull request description:
Sort media queries naturally
Fixes #761

Patch:
diff --git a/packages/core/src/index.js b/packages/core/src/index.js
index 3717da0..09aad63 100644
--- a/packages/core/src/index.js
+++ b/packages/core/src/index.js
@@ -14,9 +14,14 @@ export const merge = (a, b) => {
 // sort object-value responsive styles
 const sort = obj => {
   const next = {}
-  Object.keys(obj).sort().forEach(key => {
-    next[key] = obj[key]
-  })
+  Object.keys(obj)
+    .sort((a, b) => a.localeCompare(b, undefined, {
+      numeric: true,
+      sensitivity: 'base',
+    }))
+    .forEach(key => {
+      next[key] = obj[key]
+    })
   return next
 }
 


Test patch:
diff --git a/packages/core/test/system.js b/packages/core/test/system.js
index 969e73c..9b72cda 100644
--- a/packages/core/test/system.js
+++ b/packages/core/test/system.js
@@ -257,9 +257,10 @@ test('sorts media queries when responsive object values are used', () => {
         sm: '32em',
         md: '40em',
         lg: '64em',
+        xl: '128em',
       }
     },
-    padding: { _: 16, lg: 64 },
+    padding: { _: 16, lg: 64, xl: 128 },
     margin: { sm: 4, md: 8 },
     color: { lg: 'tomato' },
   })
@@ -267,6 +268,7 @@ test('sorts media queries when responsive object values are used', () => {
     '@media screen and (min-width: 32em)',
     '@media screen and (min-width: 40em)',
     '@media screen and (min-width: 64em)',
+    '@media screen and (min-width: 128em)',
     'padding',
   ])
 })