account for sign in double parsing

When sign was present during double parsing correctly jump forward the
extra character when parsing fraction and exponent.

components/style/attr.rs

@@ -92,17 +92,17 @@ pub fn parse_double(string: &str) -> Result<f64, ()> {
     let trimmed = string.trim_matches(HTML_SPACE_CHARACTERS);
     let mut input = trimmed.chars().peekable();
 
-    let (value, divisor) = match input.peek() {
+    let (value, divisor, chars_skipped) = match input.peek() {
         None => return Err(()),
         Some(&'-') => {
             input.next();
-            (-1f64, -1f64)
+            (-1f64, -1f64, 1)
         }
         Some(&'+') => {
             input.next();
-            (1f64, 1f64)
+            (1f64, 1f64, 1)
         }
-        _ => (1f64, 1f64)
+        _ => (1f64, 1f64, 0)
     };
 
     let (value, value_digits) = if let Some(&'.') = input.peek() {
@@ -112,11 +112,11 @@ pub fn parse_double(string: &str) -> Result<f64, ()> {
         (value * read_val.and_then(|result| result.to_f64()).unwrap_or(1f64), read_digits)
     };
 
-    let input = trimmed.chars().skip(value_digits).peekable();
+    let input = trimmed.chars().skip(value_digits + chars_skipped).peekable();
 
     let (mut value, fraction_digits) = read_fraction(input, divisor, value);
 
-    let input = trimmed.chars().skip(value_digits + fraction_digits).peekable();
+    let input = trimmed.chars().skip(value_digits + chars_skipped + fraction_digits).peekable();
 
     if let Some(exp) = read_exponent(input) {
         value *= 10f64.powi(exp)

tests/unit/style/attr.rs

@@ -14,6 +14,24 @@ fn test_parse_double() {
     }
 }
 
+#[test]
+fn test_parse_double_negative_prefix() {
+    let value = String::from("-5.6");
+    match AttrValue::from_double(value, 0.0) {
+        AttrValue::Double(_, num) => assert_eq!(num, -5.6f64),
+        _ => panic!("expected a double value")
+    }
+}
+
+#[test]
+fn test_parse_double_positive_prefix() {
+    let value = String::from("+5.6");
+    match AttrValue::from_double(value, 0.0) {
+        AttrValue::Double(_, num) => assert_eq!(num, 5.6f64),
+        _ => panic!("expected a double value")
+    }
+}
+
 #[test]
 fn test_from_limited_i32_should_be_default_when_less_than_0() {
     let value = String::from("-1");