GetBlock prefetchTxs error

[eduardfina]

Hello,

I found a couple errors while I was trying to use the getBlock function with a FallbackProvider. I'm using a FallbackProvider with 2 JsonRpcProviders, each one have Sepolia defined as a staticNetwork and an Infuria Sepolia url.

1. Even if I set prefetchTxs = true, the FallbackProvider never gets the transaction objects, #transactions is always an array of transaction hashes.
2. Using a basic JsonRpcProvider instead of the FallbackProvider I can prefetch the transactions successfully but after it when I call the getTransaction function it always duplicate the second block transaction.

For the first error I could not find an easy solution, but with this commit I solved the second one, it was not properly comparing the object transaction hash with the one we are looking for.