Maintain order of keys for Filter methods

#4

README.md

@@ -53,6 +53,10 @@ $ go get -u gopkg.in/check.v1
 History
 -------
 
+v1.5.3, 02 Jun 2015
+-------------------
+ * [Issue #4](https://github.com/magiconair/properties/issues/4): Maintain key order in [Filter()](http://godoc.org/github.com/magiconair/properties#Properties.Filter), [FilterPrefix()](http://godoc.org/github.com/magiconair/properties#Properties.FilterPrefix) and [FilterRegexp()](http://godoc.org/github.com/magiconair/properties#Properties.FilterRegexp)
+
 v1.5.2, 10 Apr 2015
 -------------------
  * [Issue #3](https://github.com/magiconair/properties/issues/3): Don't print comments in [WriteComment()](http://godoc.org/github.com/magiconair/properties#Properties.WriteComment) if they are all empty

properties.go

@@ -425,9 +425,9 @@ func (p *Properties) Filter(pattern string) (*Properties, error) {
 // for which the key matches the regular expression.
 func (p *Properties) FilterRegexp(re *regexp.Regexp) *Properties {
 	pp := NewProperties()
-	for k, v := range p.m {
+	for _, k := range p.k {
 		if re.MatchString(k) {
-			pp.Set(k, v)
+			pp.Set(k, p.m[k])
 		}
 	}
 	return pp
@@ -437,9 +437,9 @@ func (p *Properties) FilterRegexp(re *regexp.Regexp) *Properties {
 // for which the key starts with the prefix.
 func (p *Properties) FilterPrefix(prefix string) *Properties {
 	pp := NewProperties()
-	for k, v := range p.m {
+	for _, k := range p.k {
 		if strings.HasPrefix(k, prefix) {
-			pp.Set(k, v)
+			pp.Set(k, p.m[k])
 		}
 	}
 	return pp