message: use Buffer.toString('base64') instead of btoa
diff: @@ -501,7 +501,8 @@ class LocalRegtest extends BlockstackNetwork {
  method: 'sendrawtransaction',
  params: [transaction] }
  const authString =
  - btoa(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + Buffer.from(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + .toString('base64')
  const headers = new Headers({ Authorization: `Basic ${authString}` })
  return fetch(this.bitcoindUrl, { method: 'POST',
  body: JSON.stringify(jsonRPC),
  @@ -514,7 +515,8 @@ class LocalRegtest extends BlockstackNetwork {
  const jsonRPC = { jsonrpc: '1.0',
  method: 'getblockcount' }
  const authString =
  - btoa(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + Buffer.from(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + .toString('base64')
  const headers = new Headers({ Authorization: `Basic ${authString}` })
  return fetch(this.bitcoindUrl, { method: 'POST',
  body: JSON.stringify(jsonRPC),
  @@ -528,7 +530,8 @@ class LocalRegtest extends BlockstackNetwork {
  method: 'gettransaction',
  params: [txHash] }
  const authString =
  - btoa(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + Buffer.from(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + .toString('base64')
  const headers = new Headers({ Authorization: `Basic ${authString}` })
  return fetch(this.bitcoindUrl, { method: 'POST',
  body: JSON.stringify(jsonRPC),
  @@ -557,7 +560,8 @@ class LocalRegtest extends BlockstackNetwork {
  method: 'listunspent',
  params: [1, 9999999, [address]] }
  const authString =
  - btoa(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + Buffer.from(`${this.bitcoindCredentials.username}:${this.bitcoindCredentials.password}`)
  + .toString('base64')
  const headers = new Headers({ Authorization: `Basic ${authString}` })
  return fetch(this.bitcoindUrl, { method: 'POST',
  