Merge pull request #13 from keep-network/helpers

Touchups

## Add `getSignerFee` helper to Deposit. 

This line in tbtc-dapp[1] fails with the new web3 tbtc.js. The new
method call would be deposit.methods.signerFee().call(), although
the line above uses a helper - I figure we should use one here too.

## Convert redemptionCost BN to string

I found this while investigating a redemption bug reported in Discord.
I didn't test this less-travelled code path of redemption.

[1] https://github.com/keep-network/tbtc-dapp/blob/b3cfbe418cf532a3bd2747ffce1740bc41c82650/src/sagas/deposit.js#L78

src/Deposit.js

@@ -388,6 +388,14 @@ export default class Deposit {
     return toBN(await this.contract.methods.lotSizeSatoshis().call())
   }
 
+  /**
+   * Get the signer fee, to be paid at redemption.
+   * @return {Promise<BN>} A promise to the signer fee for this deposit, in TBTC.
+   */
+  async getSignerFeeTBTC() {
+    return toBN(await this.contract.methods.signerFee().call())
+  }
+
   /**
    * Returns a promise that resolves to the Bitcoin address for the wallet
    * backing this deposit. May take an extended amount of time if this deposit
@@ -715,7 +723,7 @@ export default class Deposit {
     } else {
       console.debug(`Approving transfer of ${redemptionCost} to the deposit...`)
       this.factory.tokenContract.methods
-        .approve(this.address, redemptionCost)
+        .approve(this.address, redemptionCost.toString())
         .send()
 
       console.debug(`Initiating redemption from deposit ${this.address}...`)