Fix tapos check

[c0d3ster]

Change Description

Modified the conditional in hasRequiredTaposFields to use typeof() === 'number' instead of a truthiness check for the fields. As seen in the excerpt below from the transaction header file, both of these fields are unsigned integers and should be confirmed as numbers to allow the 0 case to pass.

uint16_t               ref_block_num       = 0U; ///< specifies a block num in the last 2^16 blocks.
uint32_t               ref_block_prefix    = 0UL; ///< specifies the lower 32 bits of the blockid at get_ref_blocknum

Addresses #554

[zquestz]

Looks great to me. This fixes my issue. =)