Parse floats correctly and fix mistake from #171

The regular expression is copy & pasted form the one in the spec.
The change suggested in #171 and integrated was improper.

Closes #290.

decode_test.go

@@ -670,13 +670,13 @@ var unmarshalTests = []struct {
 		M{"ñoño": "very yes 🟔"},
 	},
 
-	// YAML Float regex shouldn't match this
+	// This *is* in fact a float number, per the spec. #171 was a mistake.
 	{
 		"a: 123456e1\n",
-		M{"a": "123456e1"},
+		M{"a": 123456e1},
 	}, {
 		"a: 123456E1\n",
-		M{"a": "123456E1"},
+		M{"a": 123456E1},
 	},
 	// yaml-test-suite 3GZX: Spec Example 7.1. Alias Nodes
 	{

resolve.go

@@ -77,7 +77,7 @@ func resolvableTag(tag string) bool {
 	return false
 }
 
-var yamlStyleFloat = regexp.MustCompile(`^[-+]?[0-9]*\.?[0-9]+([eE][-+][0-9]+)?$`)
+var yamlStyleFloat = regexp.MustCompile(`^[-+]?(\.[0-9]+|[0-9]+(\.[0-9]*)?)([eE][-+]?[0-9]+)?$`)
 
 func resolve(tag string, in string) (rtag string, out interface{}) {
 	if !resolvableTag(tag) {