Fix, Test issue of readPreferenceTags= in URI

GODRIVER-1078

If URI ends with readPreferenceTags= then we just ignore that additional
tag. Tests added ensure that this additional element of the URI is
ignored

Change-Id: Ic3e1385c98156f831055da2570a8d908d95248b6

Author: eppingere

x/network/connstring/connstring.go

@@ -546,6 +546,11 @@ func (p *parser) addOption(pair string) error {
 	case "readpreference":
 		p.ReadPreference = value
 	case "readpreferencetags":
+		if value == "" {
+			// for when readPreferenceTags= at end of URI
+			break
+		}
+
 		tags := make(map[string]string)
 		items := strings.Split(value, ",")
 		for _, item := range items {

x/network/connstring/connstring_test.go

@@ -282,10 +282,13 @@ func TestReadPreferenceTags(t *testing.T) {
 		expected []map[string]string
 		err      bool
 	}{
+		{s: "", expected: nil},
 		{s: "readPreferenceTags=one:1", expected: []map[string]string{{"one": "1"}}},
 		{s: "readPreferenceTags=one:1,two:2", expected: []map[string]string{{"one": "1", "two": "2"}}},
 		{s: "readPreferenceTags=one:1&readPreferenceTags=two:2", expected: []map[string]string{{"one": "1"}, {"two": "2"}}},
 		{s: "readPreferenceTags=one:1:3,two:2", err: true},
+		{s: "readPreferenceTags=one:1&readPreferenceTags=two:2&readPreferenceTags=", expected: []map[string]string{{"one": "1"}, {"two": "2"}}},
+		{s: "readPreferenceTags=", expected: nil},
 	}
 
 	for _, test := range tests {