Fix Pathname to String conversion error

In Rails apps the app root path is a Pathname instance. We can't use it
the same way as a String when using it in `.start_with?` when comparing
backtrace lines for error causes in
`Transaction#first_formatted_backtrace_line`.

Fix this by casting the root_path always to a String in the Config
initializer so we're always sure it's a String and can use it as such.

.changesets/fix-no-implicit-conversion-of-pathname-into-string-error.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: fix
+---
+
+Fix 'no implicit conversion of Pathname into String' error when parsing backtrace lines of error causes in Rails apps.

lib/appsignal/config.rb

@@ -212,7 +212,7 @@ def initialize(
       root_path,
       env
     )
-      @root_path = root_path
+      @root_path = root_path.to_s
       @config_file_error = false
       @config_file = config_file
       @valid = false

spec/lib/appsignal/config_spec.rb

@@ -371,6 +371,15 @@ def on_load
     end
   end
 
+  context "when root path is Pathname instance" do
+    let(:config) { described_class.new(Pathname.new("/path"), "production") }
+
+    it "converts it to a String" do
+      expect(config.root_path).to eq("/path")
+      expect(config.root_path).to be_instance_of(String)
+    end
+  end
+
   context "without config file" do
     let(:config) { described_class.new(tmp_dir, "production") }