Apply seq to sets when converting to sql

This allows them to be used as values, e.g. for in clauses, as
demonstrated in the test.

CHANGES.md

@@ -1,5 +1,6 @@
 ## 0.5.3 In development
 
+* Apply seq to sets when converting to sql (@dball)
 * Support locking selects (@dball)
 * Add sql array type and reader literal (@loganmhb)
 

src/honeysql/format.clj

@@ -320,6 +320,9 @@
       (if *subquery?*
         (paren-wrap sql-str)
         sql-str)))
+  clojure.lang.IPersistentSet
+  (-to-sql [x]
+    (-to-sql (seq x)))
   nil
   (-to-sql [x] "NULL")
   SqlParam

test/honeysql/core_test.clj

@@ -80,3 +80,13 @@
            (columns :bar)
            (values [[(honeysql.format/value {:baz "my-val"})]])
            sql/format))))
+
+(deftest test-operators
+  (testing "in"
+    (doseq [[cname coll] [[:vector []] [:set #{}] [:list '()]]]
+      (testing (str "with values from a " (name cname))
+        (let [values (conj coll 1)]
+          (is (= ["SELECT * FROM customers WHERE (id in (1))"]
+                 (sql/format {:select [:*]
+                              :from [:customers]
+                              :where [:in :id values]}))))))))