Export pageable interface to ease up mocking

The pageable interface was not exported but exposed in the client when
retrieving the next page of pageable. This made mocking pretty tough.

page.go

@@ -107,15 +107,15 @@ type SimpleShowPage struct {
 	Shows []FullShow `json:"items"`
 }
 
-// pageable is an internal interface for types that support paging
+// Pageable is an interface for types that support paging
 // by embedding basePage.
-type pageable interface{ canPage() }
+type Pageable interface{ CanPage() }
 
-func (b *basePage) canPage() {}
+func (b *basePage) CanPage() {}
 
 // NextPage fetches the next page of items and writes them into p.
 // It returns ErrNoMorePages if p already contains the last page.
-func (c *Client) NextPage(ctx context.Context, p pageable) error {
+func (c *Client) NextPage(ctx context.Context, p Pageable) error {
 	if p == nil || reflect.ValueOf(p).IsNil() {
 		return fmt.Errorf("spotify: p must be a non-nil pointer to a page")
 	}
@@ -139,7 +139,7 @@ func (c *Client) NextPage(ctx context.Context, p pageable) error {
 
 // PreviousPage fetches the previous page of items and writes them into p.
 // It returns ErrNoMorePages if p already contains the last page.
-func (c *Client) PreviousPage(ctx context.Context, p pageable) error {
+func (c *Client) PreviousPage(ctx context.Context, p Pageable) error {
 	if p == nil || reflect.ValueOf(p).IsNil() {
 		return fmt.Errorf("spotify: p must be a non-nil pointer to a page")
 	}