[Bug] Not unique exporter for exporter type

## Description

If the same exporter is defined in several levels (assembly and class) with different parameters, they are not merged.

## Steps to Reproduce

1. Download attachment
2. Decompress
3. Run
4. At the end of benchmark you can see in Console output like this:
```shell
// * Export *
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report.csv
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report-github.md
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report.html
CustomExporter - Benchmark
CustomExporter - Assembly
```

## Expected Behavior
Console output like this:
```shell
// * Export *
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report.csv
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report-github.md
BenchmarkDotNet.Artifacts\results\TestMultipleExportIssue.SimpleBenchmark-report.html
CustomExporter - Benchmark
```

Version: [0.12.1](https://github.com/dotnet/BenchmarkDotNet/releases/tag/v0.12.1)

### Attachment
[TestMultipleExportIssue.zip](https://github.com/dotnet/BenchmarkDotNet/files/6440779/TestMultipleExportIssue.zip)