Provider does not reconnect after failure

[gabi-t]

How to reproduce:

1. instantiate a web3 instance using a websockets provider (in my case - I connected it to a local parity client)
2. create a subscription to an event (in my case - newBlockHeaders)
3. kill parity
   Result: The process crashes with the following output:
   [..]node_modules/web3-core-subscriptions/src/subscription.js:279
   _this.options.requestManager.provider.reconnect();
   ^
   TypeError: _this.options.requestManager.provider.reconnect is not a function
   at Timeout._onTimeout ([..]node_modules/web3-core-subscriptions/src/subscription.js:279:67)
   at ontimeout (timers.js:488:11)
   at tryOnTimeout (timers.js:323:5)
   at Timer.listOnTimeout (timers.js:283:5)

[nivida]

This got implemented with version beta.38