test/dbi: test text and bytea types
test/data: I just noticed julia already has C_NULL defined
fix warnings about deprecated use of (implicit) convert()

The unit tests were causing the following warning to be printed:

WARNING: convert{T}(p::Type{Ptr{T}},a::Array) is deprecated, use convert(p,pointer(a)) instead.
 in convert at deprecated.jl:26
 in PQexecPrepared at /home/srp/.julia/PostgreSQL/src/libpq_common.jl:4
 in executemany at /home/srp/.julia/PostgreSQL/src/dbi.jl:211
 in test_dbi at /home/srp/.julia/PostgreSQL/test/dbi.jl:91
 in include at ./boot.jl:245
 in include_from_node1 at ./loading.jl:128
 in include at ./boot.jl:245
 in include_from_node1 at loading.jl:128
 in process_options at ./client.jl:285
 in _start at ./client.jl:354

This fixes that.

See https://github.com/JuliaLang/julia/issues/7694 for more info
take advantage of c_free() and c_realloc() support of nullptr

free(NULL) is a no-op, it will not cause a segfault or any undefined behavior

Likewise, realloc(NULL, size) is treated as if it were malloc(size)

As such, we can avoid premallocing for param_ptrs, as pgdata() will call
realloc() if it fills in that param. This is extra beneficial for strings
as we avoid allocating a small string param only to turn around and realloc
it for a larger actual string.