fix currency filter negative formatting (fix #2300)

src/filters/index.js

@@ -78,7 +78,7 @@ export default {
       : ''
     var _float = stringified.slice(-3)
     var sign = value < 0 ? '-' : ''
-    return currency + sign + head +
+    return sign + currency + head +
       _int.slice(i).replace(digitsRE, '$1,') +
       _float
   },

test/unit/specs/filters/filters_spec.js

@@ -75,9 +75,9 @@ describe('Filters', function () {
     expect(filter(undefined)).toBe('')
     expect(filter(Infinity)).toBe('')
     // negative numbers
-    expect(filter(-50)).toBe('$-50.00')
-    expect(filter(-150.43)).toBe('$-150.43')
-    expect(filter(-1500.4343434)).toBe('$-1,500.43')
+    expect(filter(-50)).toBe('-$50.00')
+    expect(filter(-150.43)).toBe('-$150.43')
+    expect(filter(-1500.4343434)).toBe('-$1,500.43')
   })
 
   it('debounce', function (done) {