[prometheus-thanos] Fix rendering when binary index headers are used

[muffix]

Is this a request for help?:
No

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
Bug report

Version of Helm and Kubernetes:
Helm: v2.16.1
Kubernetes: v1.12.8

Which chart in which version:
prometheus-thanos v4.2.0

What happened:
Following on from #316, when opting in to binary index headers, the rendered Kubernetes objects are invalid. The current version of the helm template incorrectly chomps whitespace on the right, merging the flag --experimental.enable-index-header and the next line, ports: into one: --experimental.enable-index-headerports:

What you expected to happen:
The template should generate valid Kubernetes YAML

How to reproduce it (as minimally and precisely as possible):
Set storeGateway.binaryIndexHeader.enabled to true

Anything else we need to know:
I'm going to raise a PR to fix this.