Test memory router, ensure Route onRoute is not required

[lhorie]

Currently, using <MemoryRouter> causes an error that the onRoute context property is required by <Route>.

This PR makes onRoute optional and adds tests to ensure basic usage of MemoryRouter works

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@17aa5bd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage          ?   79.35%           
=========================================
  Files             ?       10           
  Lines             ?      218           
  Branches          ?       48           
=========================================
  Hits              ?      173           
  Misses            ?       30           
  Partials          ?       15

Impacted Files	Coverage Δ
src/modules/Route.js	76.19% <100%> (ø)

---

Continue to review full report at Codecov.

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@17aa5bd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage          ?   79.35%           
=========================================
  Files             ?       10           
  Lines             ?      218           
  Branches          ?       48           
=========================================
  Hits              ?      173           
  Misses            ?       30           
  Partials          ?       15

Impacted Files	Coverage Δ
src/modules/Route.js	76.19% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 17aa5bd...5572ba5. Read the comment docs.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@17aa5bd). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage          ?   79.35%           
=========================================
  Files             ?       10           
  Lines             ?      218           
  Branches          ?       48           
=========================================
  Hits              ?      173           
  Misses            ?       30           
  Partials          ?       15

Impacted Files	Coverage Δ
src/modules/Route.js	76.19% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 17aa5bd...5572ba5. Read the comment docs.

[old-fusion-bot]

Triggered Fusion.js build verification: https://buildkite.com/uberopensource/fusion-release-verification/builds/1556