Improve error message for resolving invalid traits

Fixes [#1259]
Fixes [#1267]

In [#1156] we added support for looking up local traits with either a
symbol or string. This matches the lookup for factories and for global
traits.

We were calling `to_sym` on the trait arguments passed in when
building an object, assuming that those arguments would always respond
to `to_sym`. This leads to a confusing error message when passing an
object that doesn't respond to that method.

This PR replaces the confusing "NoMethodError: undefined method `to_sym`
for ..." with a more helpful "KeyError: Trait not registered: ...". It
ensures that all trait resolution is done using strings instead of
symbols, all objects should have a `to_s` method.

Co-authored-by: Sean Doyle <sean.p.doyle24@gmail.com>

[#1259]: https://github.com/thoughtbot/factory_bot/issues/1259
[#1267]: https://github.com/thoughtbot/factory_bot/issues/1267
[#1156]: https://github.com/thoughtbot/factory_bot/pull/1156

lib/factory_bot/definition.rb

@@ -115,7 +115,7 @@ module FactoryBot
     end
 
     def trait_for(name)
-      defined_traits.detect { |trait| trait.name == name }
+      defined_traits.detect { |trait| trait.name == name.to_s }
     end
 
     def initialize_copy(source)

lib/factory_bot/factory_runner.rb

@@ -5,7 +5,7 @@ module FactoryBot
       @strategy = strategy
 
       @overrides = traits_and_overrides.extract_options!
-      @traits    = traits_and_overrides.map(&:to_sym)
+      @traits    = traits_and_overrides
     end
 
     def run(runner_strategy = @strategy, &block)

lib/factory_bot/trait.rb

@@ -4,7 +4,7 @@ module FactoryBot
     attr_reader :name, :definition
 
     def initialize(name, &block)
-      @name = name.to_sym
+      @name = name.to_s
       @block = block
       @definition = Definition.new(@name)
 

spec/acceptance/traits_spec.rb

@@ -232,6 +232,19 @@ describe "trait indifferent access" do
   end
 end
 
+describe "looking up traits that don't exist" do
+  it "raises a KeyError" do
+    define_class("User")
+
+    FactoryBot.define do
+      factory :user
+    end
+
+    expect { FactoryBot.build(:user, double("not a trait")) }.
+      to raise_error(KeyError)
+  end
+end
+
 describe "traits with callbacks" do
   before do
     define_model("User", name: :string)

spec/support/matchers/trait.rb

@@ -1,7 +1,7 @@
 RSpec::Matchers.define :have_trait do |trait_name|
   match do |instance|
     instance.defined_traits.any? do |trait|
-      trait.name == trait_name && trait.send(:block) == @block
+      trait.name == trait_name.to_s && trait.send(:block) == @block
     end
   end