Add JSON::Builder#next_is_object_key?

spec/std/json/builder_spec.cr

@@ -307,4 +307,18 @@ describe JSON::Builder do
       builder.start_object
     end
   end
+
+  it "#next_is_object_key?" do
+    io = IO::Memory.new
+    builder = JSON::Builder.new(io)
+    builder.next_is_object_key?.should be_false
+    builder.start_document
+    builder.next_is_object_key?.should be_false
+    builder.start_object
+    builder.next_is_object_key?.should be_true
+    builder.scalar("foo")
+    builder.next_is_object_key?.should be_false
+    builder.scalar("bar")
+    builder.next_is_object_key?.should be_true
+  end
 end

src/json/builder.cr

@@ -280,6 +280,13 @@ class JSON::Builder
     end
   end
 
+  # Returns `true` if the next thing that must pushed into this
+  # builder is an object key (so a string) or the end of an object.
+  def next_is_object_key? : Bool
+    state = @state.last
+    state.is_a?(ObjectState) && state.name
+  end
+
   private def scalar(string = false)
     start_scalar(string)
     yield.tap { end_scalar(string) }