Maintain order of keys for Filter methods

https://github.com/magiconair/properties/issues/4

README.md

diff --git a/README.md b/README.md
index 3cb834c..d05d86b 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,10 @@
 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

diff --git a/properties.go b/properties.go
index 33c108c..edcaccc 100644
--- a/properties.go
+++ b/properties.go
@@ -425,9 +425,9 @@
 // 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 @@
 // 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