Stringify non-string job names in push client

We do this to all other label values in the client and we should make
this one consistent. Right now it's particularly surprising that passing
a symbol as the job name results in an error like:

```
undefined method `include?' for :foo:Symbol (NoMethodError)
```

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

Author: Sinjo

lib/prometheus/client/push.rb

@@ -87,6 +87,8 @@ def parse(url)
       end
 
       def build_path(job, grouping_key)
+        job = job.to_s
+
         # Job can't be empty, but it can contain `/`, so we need to base64
         # encode it in that case
         if job.include?('/')

spec/prometheus/client/push_spec.rb

@@ -93,6 +93,14 @@
       expect(push.path).to eql('/metrics/job/test-job/foo/bar/baz/qux')
     end
 
+    it 'converts non-string job names to strings' do
+      push = Prometheus::Client::Push.new(
+        job: :foo,
+      )
+
+      expect(push.path).to eql('/metrics/job/foo')
+    end
+
     it 'encodes the job name in url-safe base64 if it contains `/`' do
       push = Prometheus::Client::Push.new(
         job: 'foo/test-job',