Error while trying to subscribe to pendingTransactions

A clear and concise description of what the bug is.

My contract supports 3 types of events: eventA, eventB and eventC.

When i try to:

```
web3.eth
.subscribe("pendingTransactions", function(error, result) {
console.log(error);
})
```

I get the following error:

inpage.js:1 MetaMask - RPC Error: Error: SubscriptionManager - unsupported subscription type "newPendingTransactions"

The report names SubscriptionManager in the MetaMask RPC error but no file, test, or entry point. Start by reproducing the web3.eth subscription request and tracing how the pendingTransactions/newPendingTransactions type is handled. Done requires an agreed behavior for this unsupported subscription, but the issue does not state whether support, a clearer error, or documentation is expected.

Written by the indexing model from the issue text.