Replace fmt.Sprint with strconv.Itoa, specify slice capacity

Uber's Go style guide outlines a slight performance benefit when using
strconv over fmt:
https://github.com/uber-go/guide/blob/dc025303c14891f54d1847396379548773e6123e/style.md#prefer-strconv-over-fmt

Also specifiying slice capacity when it is known beforehand:
https://github.com/uber-go/guide/blob/dc025303c14891f54d1847396379548773e6123e/style.md#specifying-slice-capacity

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/gold.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"strconv"
 )
 
 // GoldService handles communication with the gold
@@ -37,7 +38,7 @@ func (s *GoldService) Give(ctx context.Context, username string, months int) (*R
 	path := fmt.Sprintf("api/v1/gold/give/%s", username)
 
 	form := url.Values{}
-	form.Set("months", fmt.Sprint(months))
+	form.Set("months", strconv.Itoa(months))
 
 	req, err := s.client.NewRequest(http.MethodPost, path, form)
 	if err != nil {