Read Slice(UInt8) as String if given as type

Fixes will#43

spec/pg/driver_spec.cr

@@ -95,6 +95,19 @@ describe PG::Driver do
     end
   end
 
+  it "treats Slice(UInt8) as String" do
+    with_db do |db|
+      db.exec "create extension if not exists citext with schema public"
+      db.exec "drop table if exists users"
+      db.exec "create table users (email citext)"
+      db.exec "insert into users values ($1)", "foo@example.com"
+
+      db.query "select email from users limit 1" do |rs|
+        assert_single_read rs, String, "foo@example.com"
+      end
+    end
+  end
+
   describe "transactions" do
     it "can read inside transaction and rollback after" do
       with_db do |db|

src/pg/result_set.cr

@@ -92,6 +92,17 @@ class PG::ResultSet < ::DB::ResultSet
     end
   end
 
+  def read(t : String.class) : String
+    value = read
+    if value.is_a?(String)
+      value
+    elsif value.is_a?(Slice(UInt8))
+      String.new(value)
+    else
+      raise "#{self.class}#read returned a #{value.class}. A String was expected."
+    end
+  end
+
   private def read_array(t : T.class) : T forall T
     col_bytesize = conn.read_i32
     if col_bytesize == -1