better handling of empty values in slices/arrays

if the slice or array has no values, skip it entirely.  This fixes
an issue where a struct field with a custom delimiter was included in
the returned url.Values with an empty string rather than being omitted
entirely (as is the case when there is no custom delimiter)

fixes #57

query/encode.go

diff --git a/query/encode.go b/query/encode.go
index 1642207..9f2a8ec 100644
--- a/query/encode.go
+++ b/query/encode.go
@@ -209,6 +209,11 @@
 		}
 
 		if sv.Kind() == reflect.Slice || sv.Kind() == reflect.Array {
+			if sv.Len() == 0 {
+				// skip if slice or array is empty
+				continue
+			}
+
 			var del string
 			if opts.Contains("comma") {
 				del = ","

query/encode_test.go

diff --git a/query/encode_test.go b/query/encode_test.go
index 8487858..a94b44d 100644
--- a/query/encode_test.go
+++ b/query/encode_test.go
@@ -144,12 +144,32 @@
 			url.Values{},
 		},
 		{
+			struct{ V []string }{[]string{}},
+			url.Values{},
+		},
+		{
+			struct{ V []string }{[]string{""}},
+			url.Values{"V": {""}},
+		},
+		{
 			struct{ V []string }{[]string{"a", "b"}},
 			url.Values{"V": {"a", "b"}},
 		},
 		{
 			struct {
 				V []string `url:",comma"`
+			}{[]string{}},
+			url.Values{},
+		},
+		{
+			struct {
+				V []string `url:",comma"`
+			}{[]string{""}},
+			url.Values{"V": {""}},
+		},
+		{
+			struct {
+				V []string `url:",comma"`
 			}{[]string{"a", "b"}},
 			url.Values{"V": {"a,b"}},
 		},