client/common: fix empty panic on set at empty capability

Author: mcuadros

clients/common/common.go

@@ -99,7 +99,10 @@ func (c *Capabilities) Get(capability string) *Capability {
 
 // Set sets a capability removing the values
 func (c *Capabilities) Set(capability string, values ...string) {
-	delete(c.m, capability)
+	if _, ok := c.m[capability]; ok {
+		delete(c.m, capability)
+	}
+
 	c.Add(capability, values...)
 }
 

clients/common/common_test.go

@@ -82,6 +82,13 @@ func (s *SuiteCommon) TestCapabilitiesSet(c *C) {
 	c.Assert(cap.Get("symref").Values, DeepEquals, []string{"bar"})
 }
 
+func (s *SuiteCommon) TestCapabilitiesSetEmpty(c *C) {
+	cap := NewCapabilities()
+	cap.Set("foo", "bar")
+
+	c.Assert(cap.Get("foo").Values, HasLen, 1)
+}
+
 func (s *SuiteCommon) TestCapabilitiesAdd(c *C) {
 	cap := NewCapabilities()
 	cap.Add("symref", "foo", "qux")